A plasma consists of a system of charged particles that exhibits collective behavior due to long range electrostatic and electromagnetic interactions between the particles. Plasmas have a variety of applications, ranging from controlled nuclear fusion to materials processing, and can affect space-craft operating in planetary orbits or in deep space.
Measurements of plasma parameters such as plasma electron density are an integral part of the characterization of plasmas. The most common diagnostic tool for this measurement is the Langmuir probe, which uses changes in charged particle flux as a function of applied voltage to measure the DC plasma impedance, which in turn can be used to determine the characteristics of such plasma parameters. See N. Hershkowitz, 1989, Plasma Diagnostics Vol. I (New York, N.Y.: Academic Press) pp. 113-121.
In spite of their common usage, Langmuir probes are of limited utility in a number of plasma environments. In many materials processing plasmas, the probe surface is modified when it is placed into the plasma, e.g., through the deposition of insulating material which prevents charge collection, or the deposition of metal, which changes the probe's surface area and in turn the measured surface current, rendering the probe ineffectual. In low-density space plasmas, Langmuir probe measurements are highly geometry and orientation-dependent due to large sheaths and the effects of magnetic fields, further limiting their utility in such cases.
To overcome these shortcomings of Langmuir probes, some researchers have used radiofrequency (RF) probes to determine plasma parameters such as plasma potential, plasma temperatures, and plasma density. See R. S. Harp and F. W. Crawford, “Characteristics of the Plasma Resonance Probe,” J. Appl. Phys. 35, 3436 (1964); R. L. Stenzel, “Microwave Resonator Probe for Localized Density Measurements in Weakly Magnetized Plasmas,” Rev. Sci. Inst. 47, 5, 603 (1976); T. Shirakawa and H. Sugai, “Plasma Oscillation Method for Measurements of Absolute Electron Density in a Plasma,” Jpn. J. Appl. Phys. 32, 5129 (1993); and D. N. Walker, R. F. Fernsler, D. D. Blackwell, and W. E. Amatucci, “On collisionless energy absorption in plasmas: Theory and experiment in spherical geometry,” Phys. Plasmas 13, 032108 (2006).
Another type of probe that has been used to measure plasma density is the hairpin resonator, which measures plasma density by noting the shift in the cavity resonance of a U-shaped wire. See R. B. Piejak, V. A. Godyak, R. Garner, and B. M. Alexandrovich “The hairpin resonator: A plasma density measuring technique revisited,” J. Appl. Phys. 95, 7 (2004). However, the hairpin resonator requires the presence of a standing wave between ends of the wire, and is thus limited in the plasma densities that it can measure.